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GEORGE E. NICHOLS Vo1 - IV > No - 2 



vegetation, the diversity in physiography may hinder or prevent the actual 

 attainment of any such vegetative uniformity. In short, when it comes to 

 the classification of plant communities in their relation to natural geographic 

 features, it becomes necessary to distinguish between geographically defined 

 areas which are determined by climate and those which are determined by 

 physiography, and between the corresponding climatically and physiographi- 

 cally determined association-complexes. 



B. Climatic Unit-areas and Climatic Plant Formations 



Climatic Unit-areas. — Differences of position in relation to latitude, oceans, 

 mountain barriers, prevailing winds, elevation, and the like, are responsible 

 for widespread differences in climate on different parts of the globe. Any 

 portion of the earth's surface which is characterized by having essentially the 

 same kind of climate throughout and whose boundaries are determined by 

 climate may be designated a climatic unit-area or climatic region : it represents 

 a unit in its relation to the climatic features of the earth as a whole. Familiar 

 examples on this continent are the " damp warm temperate " climatic region 

 of the eastern United States, the " dry steppe " climatic region of the Great 

 Plains, and the " dry desert " climatic region of the Southwest ; to which may 

 be added the various climatic belts or zones characteristic of our higher 

 mountains. 40 



Climatic Formations.— Taken in its entirety, the vegetation of any specific 

 climatic unit-area may be designated a climatic formation: it comprises a 

 complex of associations which are geographically linked with one another by 

 climate. The three climatic regions first mentioned above, for example, are 

 characterized in order by what may be termed a mesophytic deciduous forest 

 formation, a xerophytic grassland formation, and a desert formation. 



Thus defined, the climatic formation is taken as a concrete vegetation-unit ; 

 but, just as in the case of the association, this concept is capable of abstract 

 as well as concrete interpretation. Widely separated regions having similar 

 climate tend also to have similar vegetation. For example, xerophytic grass- 

 lands essentially identical in their ecological aspects to the grassland formation 

 of the Great Plains are developed in parts of Russia and China, where essen- 

 tially identical climatic conditions prevail, and the vegetation of all three areas 

 can be referred in the abstract to the same climatic formation. Again, the 

 mesophytic deciduous forest formation is developed in moist temperate regions 

 in various parts of the world. And so on. 



Distinguishing Characteristics of the Climatic Formation. — When account 

 is taken of all the many associations and association-types which go to make 

 up the vegetation in any particular climatic unit-area, it becomes apparent 



49 For a comprehensive synopsis of the climatic regions of the earth, see W. Koppen : 

 " Klassifikation der Klimate nach Temperatur, Niederschlag und Jahreslauf " (Peter- 

 manns Geog. Mitteil. 1918: 193-203, 243-248. pi. 10, 11). 



